In the early 20th century, a folklorist and statistician named Joseph Jacobs wrote that "there is about twice as much chance of finding a distinguished person among Jews as among Englishmen." Jacobs's true specialty was fairy tales, but the notion of Jewish intellectual superiority has been taken as fact by several scholars, including Charles Murray in the controversial 1994 book The Bell Curve. He and his co-author wrote that European Jews score higher on IQ tests than any other ethnic group, and Murray followed that with a 2007 Commentary article arguing that smarts were in Jewish genes.
After studying actual Jewish genes, Harry Ostrer is dubious of a strong DNA connection. Jews have been linked to many negative traits—greed, mental illness—as well as positive ones, so Ostrer in his new book, Legacy: A Genetic History of the Jewish People, explores whether genes are really involved.
In regard to intelligence, Ostrer argues (as have others) that a better explanation is that Jews were in the right place at the right time. Many studies have shown that IQ tends to rise as education systems improve over time. When the measurements trumpeted by Murray and others were done, Jews had been living at times and in places in which schools were getting better. "There still could be a genetic component," Ostrer says. "But it's not likely one that affects the whole population." He cites one genome-wide study of identical twins (who share all their genetic material) of various religions that looked for DNA that might be linked to varying intelligence. It accounted for just 0.4 percent of the effect.
Mental illness is a more complex case. Early 20th-century studies observed more diagnoses of depression and bipolar disorder in Jews than in non-Jews, but the effect might have been due to a greater tendency among Jews to seek psychiatric help. To avoid that bias, a larger study in the 1990s, sponsored by the National Institute of Mental Health, looked at Jews and non-Jews in their communities rather than in doctor's offices. Jewish men had higher rates of major depression than non-Jewish men; the rates among Jewish and non-Jewish women were equal. It's not overwhelming evidence.
And is mental illness a genetic trait? Studies comparing identical twins to nonidentical twins have found that higher rates of such illness go along with shared genes, regardless of religion or ethnicity, so DNA seems to play a role. Yet Ostrer describes two studies of Ashkenazic Jews that have failed to link any particular gene to signs of mental illness. Right now, he says, there is no basis for predicting mental illness based on genetic profiles among Jews.
He worries how studies that show correlations between traits and groups produce broader stereotypes. A higher rate of occurrence of a DNA sequence doesn't mean that the DNA, or a trait or a disease, permeates the entire population. "If 4 percent of English have a gene linked to alcoholism, and 6 percent of Irish have it, does that mean the Irish are a group of drunkards?" Not any more than genes show that Jews are geniuses, he says.